Environmental Issues > Wildlands Main Page > All Wildlands Documents

Top of Report


Chapter 5

DAMAGE TO SOIL RESOURCES AND TREE GROWTH

CHAPTER CONTENTS
Damage to soils: Logging activities, including roadbuilding, result in soil compaction, organic layer disturbance, and soil erosion, which may persist for decades.

Impacts on tree growth and health: Soil compaction and disturbance harm trees by depriving them of moisture and nutrients (through damaged roots, restricted root growth, a loss of beneficial soil microorganisms and mycorrhizal fungi, and reduced water infiltration).

Role of soil microorganisms and mycorrhizae: Soil microorganisms and mycorrhizal fungi, negatively affected by road building and logging activity, are important for nutrient cycling and the growth and survival of trees.



Damage to soils: Logging activities, including roadbuilding, result in soil compaction, organic layer disturbance, and soil erosion, which may persist for decades.


Key Finding: Logging resulted in soil compaction, displacement of surface mineral soil, loss of organic matter, and loss of nitrogen, an essential nutrient.

Source: Jurgensen, M. F., A. E. Harvey, R. T. Graham, D. S. Page-Dumroese, J. R. Tonn, M. J. Larsen and T. B. Jain. 1997. Impacts of timber harvesting on soil organic matter, nitrogen, productivity, and health of inland Northwest forests. Forest Science 43: 234-251.

The authors review scientific literature on the impact logging has on soil properties and site productivity (e.g. tree growth) in the Inland Northwest. Logging and site preparation result in soil compaction, displacement of surface mineral soil, and loss of organic matter (including woody residues and forest floor layers). Loss of soil organic matter also results in the loss of soil nitrogen and decreased ability of a site to fix new nitrogen. Soil organic matter is important for ectomycorrhizal development, which in turn plays a role in nutrient uptake and seedling survival. Studies on tree growth indicated that growth is reduced on soils affected by logging, and losses of soil organic matter and nutrients are estimated to last from 10 to 250 years, depending on site conditions.


Key Finding: Logging on volcanic ash soils in the Pacific Northwest caused soil compaction, as measured by increased soil bulk density.

Source: Geist, J. M., J. W. Hazard and K. W. Seidel. 1989. Assessing physical conditions of some Pacific Northwest volcanic ash soils after forest harvest. Soil Science Society of America Journal 53: 946-950.

The authors studied the impacts of logging on volcanic ash soils in 11 forest harvest units in the Blue Mountains of eastern Oregon and Washington (Umatilla National Forest, Malheur National Forest, and Wallowa Whitman National Forest). Sites logged 14 to 23 years earlier were compared to unlogged sites. The soil surface condition was evaluated along 30-m-long line transects, which included skid trails and landings, but excluded the primary transportation system.

Average bulk densities were higher on logged sites than unlogged sites, and frequency distributions of bulk density measurements in logged areas were skewed toward higher bulk densities. Damage due to soil compaction was evaluated at the 15% and 20% standards, i.e. when soil bulk densities were * 15% higher than the mean from unlogged areas; or soil bulk densities were * 20% higher. Under the 15% standard, 28% of the logged area had detrimental compaction. Under the 20% standard, 19% of the logged area had detrimental compaction.


Key Finding: Average soil bulk density was 15% greater on skid trails than on undisturbed soils in a ponderosa pine site 23 years after logging, and 28% greater on a lodgepole site 14 years after logging.

Source: Froehlich, H. A., D. W. R. Miles and R. W. Robbins. 1986. Growth of young Pinus ponderosa and Pinus contorta on compacted soil in central Washington. Forest Ecology and Management 15: 285-294.

The authors conducted a study to determine the effect of soil compaction on the growth of natural regeneration. The two study areas (ponderosa pine and subalpine fir) were located in southern Washington, on the eastern slope of the Cascade Mountains. Multiple regression equations were developed to determine the relationship between tree growth and the variables of age, site index, overstory basal area, number of trees per plot, bulk density, and surface organic matter.

Ponderosa pine regeneration was nine to 18 years old, growing on a ponderosa pine site selectively logged 23 years earlier. The growth of 61 ponderosa pine trees was measured and soil bulk densities were obtained. Mean soil bulk density on the skid trails was found to be 15.4% higher than on undisturbed soils. Total growth of ponderosa pine as well as growth over the last five years was significantly correlated to tree age, site index, stand basal area, and percent increase in soil bulk density. Total tree height, diameter, and volume growth were reduced by 5%, 8%, and 20% respectively at the mean increase in soil bulk density. Height and diameter growth over the previous five years showed a stronger effect of bulk density, and the authors conclude that height and diameter growth may be more affected by compaction as trees develop.

Lodgepole regeneration in the subalpine fir zone was 10 to 13 years old on a stand that had a group selection cut 14 years earlier. Twenty-five lodgepole pine trees were sampled. Mean soil bulk density was found to be 27.5% higher on disturbed than on undisturbed soils. No relationship was found between bulk density and tree growth. Age and organic matter presence were the most important variables correlated to lodgepole pine growth.


Key Finding: Compacted volcanic and granitic soils were slow to recover on skid trails in western Idaho, and after 23 years, only the bulk density of the granitic soil's top few centimeters had returned to undisturbed values.

Source: Froehlich, H. A., D. W. R. Miles and R. W. Robbins. 1985. Soil bulk density recovery on compacted skid trails in central Idaho. Soil Science Society of America Journal 49: 1015-1017.

Rates of recovery were studied on compacted skid trails on granitic soils and volcanic soils in mixed-conifer sites of west-central Idaho. Soil bulk densities were measured at 5.1-, 15.2-, and 30.5-cm depths and compared with adjacent undisturbed soil. Volcanic soils showed greater initial compaction than granitic soils. Recovery rates for the two soil types were not significantly different, however. After 23 years, only the surface 5.1 cm of granitic soil had returned to bulk density values equivalent to undisturbed values.


Key Finding: Stump removal, a method of Armillaria root disease control, resulted in high levels of soil compaction in ash-cap soils.

Source: Page-Dumroese, D. S., A. E. Harvey, M. F. Jurgensen and M. P. Amaranthus. 1998. Impacts of soil compaction and tree stump removal on soil properties and outplanted seedlings in northern Idaho, USA. Canadian Journal of Soil Science. 78: 29-34.*

The authors examined the impact of stump removal, a method that is sometimes used to control Armillaria root disease in the western United States. Control (no compaction), stump-extraction treatments, and severe compaction treatments were investigated on a site in northern Idaho with an ash-cap soil. Soil bulk density increased by 15-20% to a depth of 30 cm in the compaction treatment. Stump removal increased bulk density at the 20-30 cm depth, though surface soil bulk density decreased. Soil strength increased at the 40-45 cm depth after stump removal and was similar to values in the compacted treatment. This increase in soil strength was believed to be due to equipment vibration because volcanic ash soils are apparently particularly susceptible to vibrational compaction. The authors report that increased soil strength has been shown to impede root growth.

A year after planting, Douglas-fir seedlings had lower root volume in the soil compaction treatment. Soil compaction and stump removal also resulted in decreased ectomycorrhizal development and non-ectomycorrhizal short roots on Douglas-fir seedlings. After stump removal, there was a 70% decline in numbers of ectomycorrhizal root tips and a 63% reduction in morphological types on Douglas-fir compared to the no-compaction treatments. After three years, the height of Douglas-fir seedlings in the stump-removal plot was 20% lower and root collar diameter was 30% lower than for seedlings in the other treatments.

Western white pine seedling root volumes were not affected. They had smaller root collar diameters in the stump removal and compaction treatments but greater height in the compaction treatment. Western white pines' numbers of non-ectomycorrhizal root tips decreased significantly, although ectomycorrhizal diversity was not affected. Seedlings' ability to capture site resources was considered to be negatively affected.

* See also key finding in Chapter 2.


Key Finding: Soil bulk density increased, aeration porosity decreased, and water conductivity decreased in the upper layers of soil after logging in the Piedmont region.

Source: Gent Jr., J. A., R. Ballard, A. E. Hassan and D. K. Cassel. 1984. Impact of harvesting and site preparation on physical properties of Piedmont forest soils. Soil Science Society of America Journal 48: 173-177.

A study was conducted in a 22-year-old loblolly pine (Pinus taeda) plantation in Vance County, North Carolina, before and after logging. Soils were sandy loam grading down to clay loam at a depth of 23-30 cm. Plots were established on primary skid trails and on areas that were whole-tree harvested. Soil core samples were collected at varying depths, down to 30 cm, and soil bulk density, aeration porosity, and saturated hydraulic conductivity were measured. Soil bulk density increased by 36% in the top soil layer (0-8 cm) of skid trails and by 20% in the 8 to 15 cm layer. The new bulk densities were 1.52 mg/m3 and 1.8 mg/m3 respectively, which the authors believe may be high enough to impede root growth. In whole-tree plots, bulk density increased by 17% in the top 8 cm and by 7% in the 8 to 15 cm layer. The authors noted that bulk density increases in the top 8 cm could be high enough to impede root growth.

Aeration porosity significantly decreased at depths from the surface horizon to a depth of 23 cm on both skid trails and in whole-tree harvest areas. Saturated hydraulic conductivity significantly decreased down to depths of 8 cm on skid trails and down to depths of 15 cm in whole-tree plots. This affects the rate at which water can move through the soil system. The authors report that critical values for conductivity have not been established. At depths of 23-30 cm in whole tree plots, saturated hydraulic conductivity increased.


Key Finding: Logging was documented to cause soil compaction in a variety of soil types in the southern United States.

Source: Reisinger, T. W., G. L. Simmons and P. E. Pope. 1988. The impact of timber harvesting on soil properties and seedling growth in the South. Southern Journal of Applied Forestry 12: 58-67.

The authors review research done since 1970 on the effects of logging on soil properties and productivity in a variety of soil types in the southern United States - in the Piedmont, Coastal Plain, and mountainous areas. They report great variability in the severity of soil disturbance and compaction depending on the equipment used, soil type, and soil moisture content. However, measures of compaction such as bulk density, porosity, and hydraulic conductivity were affected in all studies reviewed. For example, bulk density increases of 10%, 13%, and 20% were reported. Aeration porosity was documented to decrease by 49% in silt loam soils and by an average of 68% in loamy sands and silty clay loams. Infiltration rates also decreased in compacted soils, and a study in the Lower Coastal Plain indicated that mean infiltration rates were down to 10-22% of original rates. Skid trails and yarding corridors reportedly suffered the greatest soil compaction.

The authors also report numerous studies that indicate decreased seedling establishment and growth on compacted soils. While some compaction effects may be ameliorated over the years, research shows that compaction may persist in the lower soil horizons.


Key Finding: Logging resulted in soil compaction and disturbance of organic matter in three New England forests.

Source: Martin, C. W. 1988. Soil disturbance by logging in New England - review and management recommendations. Northern Journal of Applied Forestry 5: 30-34.

The extent and severity of soil disturbance and compaction was assessed on three whole-tree harvest sites - a central hardwood site in Connecticut, a northern hardwood site in New Hampshire, and a spruce-fir site in Maine. At all three sites, 48-81% of the soil had some compaction. Compaction was potentially serious on 23-35% of the sites, and even one pass of a crawler-tractor was shown to reduce soil macropore space. Soil disturbance included disruption or removal of organic layers, compression ruts, and mounding of soil.


Key Finding: Logging activity caused significant soil erosion in the Pacific Northwest.

Key Finding: Soil compaction leads to surface erosion.

Key Finding: Clearcutting and post-logging slash burning were associated with high rates of ravel (upslope erosion) on various soil types in the Northwest.

Key Finding: Roads caused debris slides in areas that would be relatively stable otherwise.

Source: Swanson, F. J., J. L. Clayton, W. F. Megahan and G. Bush. 1989. Erosional processes and long-term site productivity. pp. 67-81 in Maintaining the Long-Term Productivity of Pacific Northwest Forest Ecosystems. D. A. Perry, R. Meurisse, B. Thomas, R. Miller, J. Boyle, J. Means, C.R. Perry, R. F. Powers, eds. Timber Press, Portland, Oregon.

The authors review various soil erosion processes and their impact on site productivity in the Pacific Northwest. Clearcutting and road construction have been reported to result in surface erosion and increase the frequency and extent of debris slides. The authors pinpoint surface erosion as the most likely to affect site productivity because of the large areas that can be affected and the fact that it is the most nutrient-rich layers that are lost due to soil erosion. Comparative studies showed that surface erosion was significantly higher in clearcuts than in undisturbed forest. A study in Oregon demonstrated that five years after clearcutting, surface erosion rates on a steep slope were stabilized, but still three times higher than in the forested control. Erosion was even higher in clearcut and burned forest. The authors also report overland water flow and erosion being common where soils were compacted.

Rates of soil ravel are high after slash burning on loamy soils, clay loams, and gravel loam soils in the Oregon coast range, particularly on slopes over 60%. Skeletal soils, widespread in the Klamath Mountains and Idaho Batholith are also very susceptible to ravel erosion, and soil removal could progress upslope by 50 m or more from a road cut, small slide scar or stream. The greatest loss reported was in areas with clearcutting and hot slash burning.

The impacts of forest roads on site productivity through erosion was variable according to studies on the issue. Areas downslope of roads were affected to varying degrees by the changes in drainage, depending on species' moisture requirements. Roads can also affect upslope and downslope areas by causing debris slides and modifying the groundwater system. Debris slides due to roads can apparently occur on areas that otherwise would have little chance of sliding. These slides can experience further erosion from road runoff.

The authors note that little comparative information was available on the impact of wildfires on erosion. They also state that the long-term effects of accelerated erosion are unknown at present and urged further study.


Key Finding: The likelihood of surface runoff increased on the compacted soils of skid trails.

Source: Gardner, B. D. and S. K. Chong. 1990. Hydrologic responses of compacted forest soils. Journal of Hydrology 112: 327-334.

Soil hydrologic properties were measured at a logging site in Shawnee National Forest, Illinois. The average slope of the logging trail was 19%. Soil cores were collected from undisturbed areas adjacent to the trail and from the skid trail. Bulk density, sorptivity, and hydraulic conductivity were obtained from the cores. These values were used to calculate incipient ponding time, which is used as a parameter for measuring runoff potential of an area. The authors' results show that ponding time decreased from 34.3 seconds to 19.1 and 5.8 seconds in log ruts and wheel tracks, respectively, and that runoff potential correspondingly increases.


Key Finding: Subsurface flow converted to surface flow by road cuts could trigger soil erosion and mass movement.

Source: Megahan, W. F. 1972. Subsurface flow interception by a logging road in mountains of Central Idaho. pp. 350-356 in Watersheds in Transition. Proceedings of a symposium on "Watersheds in Transition." S. C. Csallany, T. G. McLaughlin and W. D. Striffler, eds. Fort Collings, Colorado. June 19-22, 1972. AWRA. Urbana, Illinois.

The author's study site was located in the Idaho Batholith, where water in undisturbed forest rarely flows overland after a heavy rainstorm or snowmelt, but instead is primarily subsurface flow. Megahan's study measured the volume of subsurface flow intercepted by a road in two undisturbed micro-watersheds. The forest was composed primarily of ponderosa pine, Douglas-fir, and Engelmann spruce. Slopes ranged from 35% to more than 70%.

Water was collected at road cut banks where bedrock was exposed. Subsurface flow emerged on the face of the bedrock and ran down to a collection trough. Subsurface flow was therefore converted to surface flow. The author calculated that along a given length of road, the amount of subsurface flow intercepted by the road was 7.3 times greater than surface runoff from the road alone after a precipitation event.

The author discusses the impacts of converting subsurface flow to surface flow. Total watershed runoff volume probably increases. Surface flow commonly causes significant surface erosion, and excess soil water can result in mass erosion. In addition, the author mentions the potential broader ecological impacts of rerouting subsurface flow from downslope habitat, such as the alteration of vegetation species composition and growth rates.


Key Finding: Soil erosion rates due to debris slides were many times higher on forests with roads, landings, and logging activity than on undisturbed forests.

Source: Amaranthus, M. P., R. M. Rice, N. R. Barr and R. R. Ziemer. 1985. Logging and forest roads related to increased debris slides in southwestern Oregon. Journal of Forestry 83: 229-233.

The authors inventoried mass erosion events occurring over a 20-year period in the Siskiyou National Forest in the Klamath Mountains of southwestern Oregon. Aerial photos were analyzed from 24 forest sites and erosion attributed to roads, logging, or natural events. The volume of soil mass movements was estimated from the photographs, with partial field checking to confirm accuracy. Debris slides were found to be the primary type of mass erosion, accounting for about 80% of the volume of soil moved and 90% of mass erosion events inventoried. A total of almost 1.5 million yd3 of debris slide erosion occurred. Roads, occupying 2% of the area studied, were the sites for more than half the slides and 60% of the erosion volume. Clearcut areas, occupying 10% of the area studied, were the sites for 34% of the slide events and 18% of the slide volume. The authors also analyzed slides with respect to position on slopes, aspect, precipitation, and geology of study area.


Key Finding: Roads were responsible for 61% of the soil volume displaced by erosion in northwestern California.

Source: McCashion, J. D. and R. M. Rice. 1983. Erosion on logging roads in northwestern California: How much is avoidable? Journal of Forestry 81: 23-26.

The authors investigated erosion due to forest roads and logging in northwestern California. Their inventory covered 344 miles of roads in the Coast and Klamath Mountains. Roads were thinly rocked, graveled, or heavily rocked and regularly maintained logging roads. Slope, grade, aspect, cut-and-fill height, and soil volume displaced by erosion were recorded on each 1-mile road segment.

Mass erosion was the predominant form of erosion occurring in the study sites. Roads caused 152 of the 171 major erosional events inventoried (events that displaced more than 20 cubic yards of soil), and 61% of the soil volume displaced by erosion was due to these road-related events. The remainder was due to natural events and some logging-caused erosion. Road-related erosion increased with the slope traversed by the road. Seasonal roads had similar erosion rates to main-haul (and regularly maintained) roads.

In a separate study, erosion due to roads relative to logging areas was studied in 30,000 acres of commercial timberland in Six Rivers National Forest. The road network occupied less than 4% of the total logging area. Total erosion from the 30,000 acres was 137,800 cubic yards. Of this total, 40% came from the roads and 60% from the logged areas. The average erosion rate in the road rights-of-way (47 cubic yards per acre) was 17 times the average erosion rate in the logging areas (2.82 cubic yards per acre).


Key Finding: Clearcutting increased the frequency of mass soil movements from hillsides.

Source: Gray, D. H. 1970. Effects of forest clear-cutting on the stability of natural slopes. Bulletin of the Association of Engineering Geologists 7: 45-66.

A review of the scientific literature, including research from Alaska, Utah, California, Oregon, and Japan, demonstrates that clearcutting on slopes increased the frequency of mass soil movement events (landslides, earthflows, slips, etc.). The loss of forest cover is believed to affect slope stability in two principal ways:

1) Mechanical root support due to interconnected root systems is lost after logging. Research in Alaska, for example, indicated a time lag after clearcutting before landslide activity increased and a lack of landslide correlation with rainfall intensity. The authors believe this is due to the increased deterioration of root systems with time. Other studies similarly show that with increasing age and maturity, the effectiveness of forest cover in preventing landslides increases.

2) A denuded slope is likely to reach critical soil saturation earlier than a forested slope (since no transpiration from trees can occur). Therefore, during a large storm, it is predicted that these soils will reach a critical failure condition earlier than a forested slope would.



Impacts on tree growth and health: Soil compaction and disturbance harm trees by depriving them of moisture and nutrients (through damaged roots, restricted root growth, a loss of beneficial soil microorganisms and mycorrhizal fungi, and reduced water infiltration).


Key Finding: Soil compaction results in root damage and decreased root growth, which decrease plants' ability to access nutrients and water.

Key Finding: Soil compaction and organic matter disturbance cause a decline in mycorrhizal fungi.

Key Finding: Soil compaction results in reduced infiltration rates and increased surface erosion.

Key Finding: Soil compaction results in a loss in site productivity as measured by tree growth.

Source: Childs, S. W., S. P. Shade, D. W. R. Miles, E. Shepard and H. A. Froehlich. 1989. Soil physical properties: importance to long-term forest productivity. pp. 53-66 in Maintaining the Long-Term Productivity of Pacific Northwest Forest Ecosystems. D. A. Perry, R. Meurisse, B. Thomas, R. Miller, J. Boyle, J. Means, C.R. Perry, R. F. Powers, eds. Timber Press, Portland, Oregon.

Damage to soil resources through soil compaction, surface disturbance, and topsoil loss is reviewed. Soil compaction results in decreased soil aeration, decreased water storage, and an increase in soil bulk density. A corresponding decrease in root growth as well as damage to roots results in a decreased ability of plants to access nutrients and water. Mycorrhizal fungi and soil microbial populations are diminished by compaction and organic matter disturbance. Nutrient availability to plants is reduced. Water infiltration rates decrease, and erosion losses of valuable topsoil layers increase. Changes in infiltration and corresponding changes in the hydrologic cycle can also degrade the site. Several studies report the effects of soil compaction as being at least several decades long.

The result of these management impacts on the soil is a loss in site productivity, such as tree height and volume. The authors also caution that the resilience of the ecosystem may be negatively affected, with a corresponding increased susceptibility to damage in the future.


Key Finding: Soil compaction restricted root growth and increased moisture stress in southern U.S. forests.

Source: Reisinger, T. W., G. L. Simmons and P. E. Pope. 1988. The impact of timber harvesting on soil properties and seedling growth in the South. Southern Journal of Applied Forestry 12: 58-67.*

The authors review research done since 1970 on the effects of logging on soil properties and productivity in a variety of soil types in the southern United States - in the Piedmont, Coastal Plain, and mountainous areas. They reported great variability in the severity of soil disturbance and compaction based on factors such as the equipment used, soil type, and soil moisture content. However, measures of compaction such as bulk density, porosity, and hydraulic conductivity were affected in all studies. For example, bulk density increases of 10%, 13%, and 20% were reported. Aeration porosity was documented to decrease by 49% in silt loam soils and by an average of 68% in loamy sands and silty clay loams. Infiltration rates also decreased in compacted soils, and a study in the Lower Coastal Plain indicated that mean infiltration rates were down to 10-22% of original rates. Skid trails and yarding corridors reportedly suffered the greatest soil compaction.

The authors also report numerous studies that indicate decreased seedling establishment and growth on compacted soils. While some compaction effects may be ameliorated over the years, research shows that compaction may persist in the lower soil horizons.

* See also key finding above.


Key Finding: Soil compaction after logging resulted in a loss of soil pore space and a 33% reduction in water to plants.

Key Finding: Soil compaction by logging reduced the movement of water through the soil (saturated hydraulic conductivity), with increases in runoff predicted.

Source: Purser, M. D. and T. W. Cundy. 1992. Changes in soil physical properties due to cable yarding and their hydrologic implications. Western Journal of Applied Forestry 7: 36-39.

Soil bulk density and saturated hydraulic conductivity (the rate at which water can move through the soil) were measured on skid trails before and after logging in the northern Cascade Mountains, Washington. The study site, a western hemlock/Pacific silver fir/western red cedar forest, was located on a steep slope, with gradients ranging from 22* to 45*. Surface and subsurface soils were sampled on four transects.

Mean bulk density values were significantly higher after logging than before. Mean of the saturated hydraulic conductivity values (Ks) was significantly lower after logging. Saturated hydraulic conductivity values (Ks) were significantly lower after logging on three of the four transects. On the fourth site, located in a hollow, sampling locations fell primarily on soil trapped behind slash, rather than the native soil, and Ks values increased.

Increased bulk density and the change in available soil pore space and hydraulic conductivity can affect water flow after rainfall events and result in surface overland flow. Surface overland flow and subsurface flow were calculated for different rainfall depths and increased runoff was predicted in the postlogging soils.


Key Finding: Soil compaction reduced growth of young ponderosa pine.

Source: Froehlich, H. A., D. W. R. Miles and R. W. Robbins. 1986. Growth of young Pinus ponderosa and Pinus contorta on compacted soil in central Washington. Forest Ecology and Management 15: 285-294.*

The authors conducted a study to determine the effect of soil compaction on the growth of natural regeneration. The two study areas (ponderosa pine and subalpine fir) were located in southern Washington, on the eastern slope of the Cascade Mountains. Multiple regression equations were developed to determine the relationship between tree growth and the variables of age, site index, overstory basal area, number of trees per plot, bulk density, and surface organic matter.

Ponderosa pine regeneration was nine to 18 years old, growing on a ponderosa pine site selectively logged 23 years earlier. The growth of 61 ponderosa pine trees was measured and soil bulk densities were obtained. Mean soil bulk density on the skid trails was found to be 15.4% higher than on undisturbed soils. Total growth of ponderosa pine as well as growth over the last five years were significantly correlated to tree age, site index, stand basal area, and percent increase in soil bulk density. Total tree height, diameter, and volume growth were reduced by 5%, 8%, and 20% respectively at the mean increase in soil bulk density. Height and diameter growth over the previous five years showed a stronger effect of bulk density, and the authors concluded that height and diameter growth may be more affected by compaction as trees develop.

Lodgepole regeneration, in the subalpine fir zone, was 10 to 13 years old, on a stand that had a group selection cut 14 years earlier. Twenty-five lodgepole pine trees were sampled. Mean soil bulk density was found to be 27.5% higher on disturbed than on undisturbed soils. No relationship was found between bulk density and tree growth. Age and organic matter presence were the most important variables correlated to lodgepole pine growth.

* See also key finding above.


Key Finding: Beneficial soil microorganisms and mycorrhizal fungi occur primarily in soil organic layers. Soil compaction and the disturbance of organic layers of the soil due to logging activities alter soil microbial activity and adversely affect mycorrhizal populations.

Source: Amaranthus, M. P., J. M. Trappe and R. J. Molina. 1989. Long-term forest productivity and the living soil. pp. 36-52 in Maintaining the Long-Term Productivity of Pacific Northwest Forest Ecosystems. D. A. Perry, R. Meurisse, B. Thomas, R. Miller, J. Boyle, J. Means, C.R. Perry, R. F. Powers, eds. Timber Press, Portland, Oregon.

The authors review research on the functions of soil organisms, their role in site productivity, and the impacts of forest management. Organic layers, woody debris, and the upper mineral soil are the primary substrate for soil biological activity.

Soil organisms are critical for nutrient cycling, including processes such as decomposition, nutrient storage, and nitrogen fixing. Mycorrhizal fungi, important for most timber species, increase the uptake of nutrients and water. They have also been shown to protect tree species from pathogens such as Phytophthora cinnamomi, Fusarium oxysporum, and Rhizoctonia solani. Soil structure - the stability and size of pores - is maintained by mycorrhizae and other soil microbes.

Logging and site preparation have the greatest impact on soil organisms. By destroying large pores in the soil (important for oxygen and water movement), soil compaction drastically changes microbial activity. Loss of organic layers of the soil adversely affects ectomycorrhizae, which are primarily in these layers. Erosion and loss of topsoil have been shown to result in a loss of mycorrhizae and a decline in site productivity.


Key Finding: Ectomycorrhizal abundance and diversity on Douglas-fir seedlings were much lower in soils compacted by stump removal than in undisturbed soils.

Source: Page-Dumroese, D. S., A. E. Harvey, M. F. Jurgensen and M. P. Amaranthus. 1998. Impacts of soil compaction and tree stump removal on soil properties and outplanted seedlings in northern Idaho, USA. Canadian Journal of Soil Science 78: 29-34.*

The authors examine the impact of stump removal, a method that is sometimes used to control Armillaria root disease in the western United States. Control (no compaction), stump-extraction treatments, and severe compaction treatments were investigated on a site in northern Idaho with an ash-cap soil. Soil bulk density increased by 15-20% to a depth of 30 cm in the compaction treatment. Stump removal increased bulk density at the 20-30 cm depth, though surface soil bulk density decreased. Soil strength increased at the 40-45 cm depth after stump removal and was similar to values in the compacted treatment. This increase in soil strength was believed to be due to equipment vibration because volcanic ash soils are apparently particularly susceptible to vibrational compaction. The authors report that increased soil strength has been shown to impede root growth.

A year after planting, Douglas-fir seedlings had reduced root volume in the soil compaction treatment. Soil compaction and stump removal also resulted in decreased ectomycorrhizal development and non-ectomycorrhizal short roots on Douglas-fir seedlings. After stump removal, there was a 70% decline in numbers of ectomycorrhizal root tips and a 63% reduction in morphological types on Douglas-fir compared to the no-compaction treatments. After three years, the height of Douglas-fir seedlings in the stump-removal plot was 20% lower and root collar diameter was 30% lower than for seedlings in the other treatments.

Western white pine seedling root volumes were not affected. They had smaller root collar diameters in the stump-removal and compaction treatments but greater height in the compaction treatment. Western white pines' numbers of non-ectomycorrhizal root tips decreased significantly, although ectomycorrhizal diversity was not affected. Seedlings' ability to capture site resources was considered to be negatively affected.

* See also key findings in Chapter 2 and above.


Key Finding: A 20% increase in soil bulk density due to soil compaction significantly reduced the numbers of root tips on Douglas-fir and western white pine seedlings.

Key Finding: Ectomycorrhizal root tip abundance and diversity in Douglas-fir seedlings were decreased by soil compaction and organic layer removal.

Source: Amaranthus, M. P., D. Page-Dumroese, A. Harvey, E. Cazares and L. F. Bednar. 1996. Soil compaction and organic matter affect conifer seedling nonmycorrhizal and ectomycorrhizal root tip abundance and diversity. Research Paper PNW-RP-494. USDA Forest Service. Pacific Northwest Research Station. 12 p.

The influence of organic matter removal and soil compaction was measured on Douglas-fir and western white pine seedlings. For Douglas-fir, moderate and severe soil compaction significantly reduced nonmycorrhizal root tip abundance. Ectomycorrhizal root tip abundance on Douglas-fir was reduced by 60% in plots with severe compaction and organic matter removal. In plots with severe compaction but no organic matter removed, ectomycorrhizal diversity was significantly reduced, with the average number of ECM types decreasing from 2.7 to 1.

For western white pine, ectomycorrhizal diversity was not affected by soil compaction or organic matter removal treatment. The number of ectomycorrhizal root tips were also not significantly affected by either kind of treatment. However, moderate and severe soil compaction significantly reduced nonmycorrhizal root tip abundance.

The authors discuss the impact of increased soil density on nonmycorrhizal root tips and indicate that a 20% increase in bulk density could significantly reduce nonmycorrhizal root tip numbers. They note that these declines could decrease the capture of site resources by seedlings. Similarly, based on studies on site productivity, the authors report that mycorrhizae are important for seedling establishment after a disturbance. Mycorrhizal diversity may also be important for forest resilience, since each type has a different function.


Key Finding: Soil erosion results in the loss of nutrients and water availability, degraded soil structure, and the loss of important soil organisms including mycorrhizal fungi.

Key Finding: Erosion of the topmost soil layers, which are the most important for nutrients, water, and soil biota, is the most damaging to site productivity.

Key Finding: Roads in mountainous areas affected site productivity upslope and downslope of the road through changes in the groundwater system and through debris slides.

Source: Swanson, F. J., J. L. Clayton, W. F. Megahan and G. Bush. 1989. Erosional processes and long-term site productivity. pp. 67-81 in Maintaining the Long-Term Productivity of Pacific Northwest Forest Ecosystems. D. A. Perry, R. Meurisse, B. Thomas, R. Miller, J. Boyle, J. Means, C.R. Perry, R. F. Powers, eds. Timber Press, Portland, Oregon.*

The authors review various soil erosion processes and their impact on site productivity in the Pacific Northwest. Clearcutting and road construction have been reported to result in surface erosion and to increase the frequency and extent of debris slides. The authors pinpoint surface erosion as the most likely to affect site productivity because of the large areas that can be affected and the fact that it is the most nutrient-rich layers that are lost due to soil erosion. Comparative studies showed that surface erosion was significantly higher in clearcuts than in undisturbed forest. A study in Oregon demonstrated that five years after clearcutting, surface erosion rates on a steep slope were stabilized, but still three times higher than in the forested control. Erosion was even higher in clearcut and then burned forest. The authors also report overland water flow and erosion being common where soils were compacted.

Rates of soil ravel have been reported to be high after slash burning on loamy soils, clay loams, and gravel loam soils in the Oregon coast range, particularly on slopes over 60%. Skeletal soils, widespread in the Klamath Mountains and Idaho Batholith, were also very susceptible to ravel erosion, and soil removal could progress upslope by 50 m or more from a road cut, small slide scar or stream. The greatest loss was in areas with clearcutting and hot slash burns.

The impacts of forest roads on site productivity through erosion was variable according to studies on the issue. Areas downslope of roads were affected to varying degrees by the changes in drainage, depending on species' moisture requirements. Roads can also affect upslope and downslope areas by causing debris slides and modifying the groundwater system. Debris slides due to roads can apparently occur on areas that otherwise would have little chance of sliding. These slides can experience further erosion from road runoff. The authors note that little comparative information was available on the impact of wildfires on erosion. They also stated that the long-term effects of accelerated erosion are unknown at present, and urged further study.

* See also key finding above.



Role of soil microorganisms and mycorrhizae: Soil microorganisms and mycorrhizal fungi, negatively affected by road building and logging activity, are important for nutrient cycling and the growth and survival of trees.


Key Finding: Healthy ectomycorrhizal populations are important for forest stability and recovery after a disturbance.

Source: Amaranthus, M. P. and D. A. Perry. 1994. The functioning of ectomycorrhizal fungi in the field: linkages in space and time. Plant and Soil 159: 133-140.

The authors review the importance of ectomycorrhizal fungi (ECM) to the growth and survival of trees - they take up nutrients and water, extend feeder root longevity, protect against pathogens, maintain soil structure, and can protect plants from toxic heavy metals. Furthermore, studies document that roots of different plants can be linked by commonly shared ECM fungi. Mycorrhizal hyphae supported by one plant can aid in the establishment of another plant. As a result, young seedlings can form mycorrhizae and obtain energy from an already established host tree. Extending mycelium may also help speed up regeneration in adjacent small forest openings. The authors note that ECM fungi may play a critical role during disturbance when the above-ground community dramatically changes. The existing fungi form a link between the old and new stands by aiding in the establishment of new host trees. Studies showed that tree seedling establishment was much less successful in sites without the appropriate mycorrhizae, such as on sites invaded by non-native plants, which are usually non-mycorrhizal or are associated with different mycorrhizal species.


Key Finding: Mycorrhizal fungi increase nutrient uptake in plants.

Source: Marschner, H. and B. Dell. 1994. Nutrient uptake in mycorrhizal symbiosis. Plant and Soil 159: 89-102.

The authors review the role of mycorrhizae in plant nutrition and the status of research on this topic. Mycorrhizae can increase nutrient uptake by increasing the surface adsorbing area, by excreting compounds that help take up immobile nutrients, or by modifying the soil microflora. Some ectomycorrhizal species, for example, release oxalic acid, which can mobilize phosphorus in calcareous soils where it would otherwise be sparingly soluble. Ectomycorrhizae in particular have been shown to assist in the acquisition of phosphorus, nitrogen, and potassium. There are few definite studies apparently on the role of ectomycorrhizae in the acquisition of other nutrients, particularly micronutrients. However, studies on ectomycorrhizae have also shown that they protect their host plants from excessive uptake of copper and zinc in soils high in heavy metals.


Key Finding: A healthy population of soil organisms is critical for nutrient cycling.

Key Finding: Mycorrhizae increase the uptake of nutrients and water.

Source: Amaranthus, M. P., J. M. Trappe and R. J. Molina. 1989. Long-term forest productivity and the living soil. pp. 36-52 in Maintaining the Long-Term Productivity of Pacific Northwest Forest Ecosystems. D. A. Perry, R. Meurisse, B. Thomas, R. Miller, J. Boyle, J. Means, C.R. Perry, R. F. Powers, eds. Timber Press, Portland, Oregon. *

The authors review research on the functions of soil organisms, their role in site productivity, and the impacts of forest management. Organic layers, woody debris, and the upper mineral soil are the primary substrate for soil biological activity.

Soil organisms are critical for nutrient cycling, including processes such as decomposition, nutrient storage, and nitrogen fixing. Mycorrhizal fungi, important for most timber species, increase the uptake of nutrients and water. They have also been shown to protect tree species from pathogens such as Phytophthora cinnamomi, Fusarium oxysporum, and Rhizoctonia solani. Soil structure - the stability and size of pores - is maintained by mycorrhizae and other soil microbes.

Logging and site preparation have the greatest impact on soil organisms. By destroying large pores in the soil (important for oxygen and water movement), soil compaction drastically changes microbial activity. Loss of organic layers of the soil adversely affects ectomycorrhizae, which are primarily in these layers. Erosion and loss of topsoil have been shown to result in a loss of mycorrhizae and a decline in site productivity.

* See also key finding above.

Sign up for NRDC's online newsletter

See the latest issue >

Give the Gift That Will Make a Difference: Leader of the Pack

NRDC Gets Top Ratings from the Charity Watchdogs

Charity Navigator awards NRDC its 4-star top rating.
Worth magazine named NRDC one of America's 100 best charities.
NRDC meets the highest standards of the Wise Giving Alliance of the Better Business Bureau.


Donate now >

Related Stories

Q&A: Documentary Filmmaker Ken Burns on National Parks
Ken Burn spoke to OnEarth about his motivation for his new documentary series on America's national parks.
In the Canadian Boreal Forest, a Conservation Ethic at Work
After fighting successfully for years to keep destructive logging, hydropower and mining projects out of their traditional territory, the people of Poplar River are now working to secure permanent protection for their boreal forest homeland.
Shop Smart, Save Forests
Share | |
Find NRDC on
YouTube